The present invention relates to the field of automatic infusion making machines, in particular for making espresso coffee, and more particularly it concerns a piston for the infusion assembly in one such machine.
It is known that, for making espresso coffee and other beverages, automatic machines are used, which internally contain a set of components allowing an operator, by simply pressing a push-button, to make the machine perform a group of operations, in a predetermined sequence, until the espresso coffee infusion is directly delivered into a cup. Essentially, such operations are: grinding the exact amount of coffee grains, accumulating the ground coffee powder in a suitable chamber, compacting and compressing the coffee powder until forming a “tablet” with predetermined compactness and size, sending a dosed water flow at a predetermined temperature through said tablet, collecting and sending the coffee infusion downstream the powder tablet towards a group of ducts and from said ducts towards the cup, and ejecting the exhausted powder tablet towards a suitable container.
The mechanical and thermal wear the components of such a machine undergo during the operating cycle is made more severe by the extreme aggressiveness, from both the chemical and the mechanical standpoint, of the coffee powder obtained by grinding the grains.
From the chemical standpoint, indeed it is known that, during the operating cycle, the oil substances contained in toasted coffee are spontaneously released, mainly because of the high temperatures that are required to produce and maintain at about 90° C. the water necessary for delivery. From the mechanical standpoint, as known, abrasiveness of the ground coffee powder is high as a consequence of the toasted grain hardness and of the very fine grinding granulometry.
Therefore, automatic machines for making espresso coffee are to be submitted to periodical maintenance operations, with repair and replacement of the worn parts.
Different solutions have been adopted in the art to contrast the above-mentioned aggressiveness, by choosing more resistant materials and/or by various surface treatments, capable of protecting the surfaces in contact with the coffee powder.
Greater difficulties have been on the contrary experienced in dealing with the problem of the coffee powder abrasiveness, especially in respect of those components which are to ensure a perfect hydraulic tightness in the different operations during coffee powder infusion and which are therefore to keep their mechanical integrity.
Among those components, of particular importance is the gasket mounted onto the pistons which are to compact the tablet of ground coffee, to compress it in the infusion chamber, to keep a constant tightness of the chamber while pressurised hot water is being pumped for the coffee infusion, and lastly to convey the exhausted tablet outside the infusion chamber for evacuation.
Referring to FIG. 1, there is shown an infusion assembly of a prior-art coffee making machine, as disclosed for instance in EP-A-1260166. Said infusion assembly, generally denoted by reference numeral 71, comprises a horizontally moving chamber 73, sliding on guides 75 and driven by a threaded shaft 77. The chamber has an internal cavity 79, intended to receive the dose of coffee powder or other powdered substance. A first piston 81a, also horizontally moving, and a second stationary piston 81b, facing the first piston, co-operate with said moving chamber 73. Said moving chamber 73 has ducts 83 for introducing coffee powder or other powdered substance into cavity 79. Also moving piston 81a is slidably mounted on guides, and it is driven by a shaft 85 co-operating with shaft 77 driving moving chamber 73, in such a manner that moving chamber 73 and moving piston 81a are simultaneously translated in opposite directions. Moving piston 81a has a head 87a, which can fit into infusion chamber 73. Hydraulic tightness between said head and said chamber is ensured by an O-ring 89a. Said head 87a is equipped at its free end with a disc-shaped filter 91a for the coffee and it has an axial duct 26 for introducing hot water, supplied through tubes not shown. Stationary piston 81b has a head 87b equipped with a disc-shaped filter 91b for the coffee and an O-ring 89b for hydraulic tightness with infusion chamber 73. The piston is integral with a rod 93 having an axial duct intended to convey the prepared infusion outside infusion assembly 71.
During the operation cycle of the infusion assembly, chamber 73 and moving piston 81a are translated relative to each other in order to pass from a phase of coffee powder loading to a compression and infusion phase and from the latter to a phase of used coffee ejection. In the loading phase, moving chamber 73 and moving piston 81a are in such a position that cavity 79 forms an infusion chamber closed by heads 87a, 87b of pistons 81a, 81b, and the coffee powder dose can be introduced through ducts 83. Once the desired coffee dose has been introduced, the coffee powder compaction and compression phase is performed, by actuating shafts 77, 85 so that said chamber 73 and said moving piston 81a are translated by such an extent that heads 87a, 87b are brought to the minimum possible distance, and by compressing said powder between said heads. Under such conditions, the machine is ready for the introduction of infusion water. Hot water is introduced into cavity 79 through filter 91a and the infusion then flows out through filter 91b. At the end of the percolation time, shafts 77, 85 are again actuated, but in reverse directions with respect to the compression phase, thereby spacing apart heads 87a, 87b of the pistons. When shafts 77, 85 reach the end of that new displacement, infusion assembly 71 is in the condition of coffee ejection, in which cavity 79 is open on the side if moving piston 81a and the used coffee is ejected thanks to the thrust of stationary piston 81b due to the backward movement of moving chamber 73 towards said stationary piston 81b. 
Several solutions have been developed in the past to limit the O-ring wear, in particular in the piston moving into and out of the infusion chamber at each operating cycle.
According to prior art solutions, the piston head is formed in two parts and the O-ring is located between said parts, so that only when the two piston parts approach each other an O-ring expansion takes place such that O-ring sealingly adheres to the infusion chamber wall. Examples of such solutions can be found in CH570145, EP0608805 and FR2202668.
Another example of two-part piston according to the prior art is denoted by reference numeral 51 in FIG. 2.
In the illustrated example, piston 51 has a head divided into two parts 53a, 53b kept joined by retaining screws 55. A spring 57 is located between the two piston parts 53a, 53b so as to keep the two piston parts spaced apart and to allow them to approach each other only when the compression force exerted by the piston on the coffee powder exceeds the elastic resistance of the spring. An O-ring 59 is located between the two piston parts 53a, 53b in an annular groove defined by an internal cylindrical abutment and by two facing conical abutments. When the piston is in idle condition, O-ring 59 lies upon cylindrical abutment 59 and has an external circumference with substantially the same diameter as piston 51. As the two piston parts move towards each other, the two facing conical abutments cause O-ring 59 to expand towards the outside and press it against the wall of the infusion chamber.
The prior art solutions have a number of drawbacks.
First of all, constructing the piston head by using a plurality of telescopic members results in an increase of the number of components and requires using additional gaskets to ensure the hydraulic tightness between such components, in order to avoid seepage into the piston.
Moreover, the proposed arrangements do not prevent all or part of the individual components from undergoing contact with water during the infusion phase and consequently from undergoing a continuous deposition of oil substances released by the coffee powder. With continuing use, the constant accumulation of said substances entails an increased friction between the individual parts, which may lead to system blockage and consequent damages to the O-ring, until a complete breaking thereof.